1. Field of the Invention
The subject-matter of the present invention is a new method for the subsequent coloring of ceramic bodies by inserting chromophoric metal ions into host lattices in a ceramic body, as well as the ceramic bodies produced with this method.
2. Description of the Related Art
For the coloring of ceramic materials, only refractory pigments can be considered since colored glazes are fired on at temperatures of 700 to 1100.degree. C. and sanitary ceramics are fired on at 1400.degree. C. The pigments used in particular are colored oxides of iron, chromium, manganese, pure phases and mixed phases with a spinel structure, e.g. compounds of Al, Ni, Cr, Zn, Co, Cu, Mn, Fe, U and V, a number of silicates, sulfides and mixed phases with chromophoric cations in colorless host lattices of the rutile type such as TiO.sub.2, SnO.sub.2, ZrO.sub.2, ZrSiO.sub.4 and PbO.sub.2. It is possible to produce nearly all prevailing colors as refractory pigments by selecting the base materials, their concentration in the mixed phases and the production conditions. The oxidic mixed-phase pigments, which are particularly esteemed because of their brilliance, are composed of a stable, colorless oxide lattice, normally of the spinel type (Me.sup.2+ Me.sub.2.sup.3+ O.sub.4 or Me.sub.2.sup.2+ Me.sup.4+ O.sub.4 as basic formula) or the rutile type, such as TiO.sub.4, SnO.sub.2, ZrO.sub.2, ZrSiQ.sub.4, PbO.sub.2 into which the chromophoric cations of other metals that effect the color are inserted. Crystallochemically, these are mixed crystals. The rutile-type mixed oxides, which are of particular interest in this connection, generally contain nickel, cobalt, chromium, copper, manganese, iron or vanadium as chromophoric cation and antimony, niobium or tungsten as higher-valence metal ions for a valence compensation of these two-valence or three-valence ions relative to the four-valence rutile ion to be replaced. These oxidic mixed-phase pigments are produced principally by firing the respective oxidic components in the solid state at temperatures ranging from 800 to 1400.degree. C., wherein the more reactive the components used are, that is to say the more finely they are dispersed and the better they are mixed, the easier the solid-state reaction progress becomes. The starting components are frequently produced from aqueous solutions through a joint precipitation of hydroxides or carbonates. The formation temperature of the oxidic mixed-phase pigments can be reduced by adding mineralizers (e.g. lithium chloride or sodium chloride), thus making it possible to avoid the heavy sintering through forming of large pigments, which otherwise can easily occur at high temperatures (compare Ullmann's Enc. of Technical Chemistry, 4.sup.th edition (1979), Vol. 18, pp 599-628; DE-AS 19 03 755; U.S. Pat. No. 3,022,186; magazine: "Angewandte Chemie" [Applied Chemistry] 1/1962, pp 23-27 and cfi/Ber. DKG 4/1993, pp 146-148). With respect to the particularly interesting yellow pigments containing Ni and Cr as coloring ions, we want to refer to the DE-OS-24 16 347; the DE-OS-26 05 651 and the U.S. Pat. No. 2 992 123.
For the coloring of ceramics, these pigments are placed either together with glass-forming substances, the so-called frit, onto the prefired ceramic and are melted or sintered together with this ceramic (glazing or enamel), or, depending on the mechanical stress, are applied to or burnt into the glaze (so-called melt pigments or overglaze pigments) or are applied to the ceramic body and subsequently coated with a glaze (underglaze pigments). The so-called engobe technique is used to produce non-glazed ceramics, for which a refined suspension of clay minerals mixed with the pigments is applied as a thin layer to the ceramic material and is then fired on. A thorough coloring of the total ceramic material is normally ruled out because of the high cost of the pigments. The great advantage of the engobe technique is that it produces not only a thin surface coloration, but that the ceramic material is provided with a thoroughly colored surface layer, so that a rough and uneven surface can be made smooth through grinding and polishing, without removing the color. The disadvantage of this method is that in each case the total surface is coated with the colored layer and a pattern can therefore not be applied.
In order to apply color patterns to non-glazed ceramics, attempts have been made for some time to apply at a later date solutions of chromophoric metal compounds onto the fired ceramic surface, so that these can penetrate into the surface and will result in a surface layer interspersed with the colored metal oxides that form, following the drying and burning process. For a pink coloration, compare the DE-OS 195 46 325 and for a black coloration the DE-OS 196 25 236. Even though the number of coloring options based on this variation of the technique is high and a higher number of colors can be produced, the range of colors is limited through the limitation to oxidic colors. It is not possible in this way to produce colors with mixed-oxide pigments of the rutile and spinel type because it would not be possible to dissolve sufficient concentrations of the various starting minerals side-by-side and such that they are stable in uniform solutions and because the suspensions with finished pigments do not penetrate deeply enough into the ceramic surface.
In accordance with the DE 31 09 927, the salts or oxides of various chromophoric metals as such are applied in a printing process to the ceramics and produce the corresponding color each by itself. A joint application is also addressed there, but more along the lines of forming mixed colors or applying them side-by-side to create corresponding colored pictures. Example 2 describes the application of a layer of zirconium oxide glaze. However, the rutile pigments cannot form here since, on the one hand, only one metal ion is applied respectively (copper or cobalt), which is not sufficient for the mixed-phase formation and, on the other hand, the ZrO.sub.2 in zirconium oxide glazes is not contained in defined crystals with a rutile structure, but as undercooled, glass-type melt in a mixture with the other glazing components.
The object therefore was to find a new method for producing oxidic mixed-phase pigments in a ceramic surface layer, wherein the method also allows a partial coloring of the surface layer.